1. Field of the Invention
The present invention relates to a DC offset canceling circuit applied in a variable gain amplifier, and particularly to a DC offset canceling circuit which utilizes a chopper stabilization method to cancel DC offset of output stage.
2. Description of the Related Art
Variable gain amplifiers (VGA), which amplify input signal to necessary voltage levels in a system in demodulation process, are widely used in home network transceivers which transmit signals via cable. When the variable gain amplifier is used, a differential input end of an internal operational amplifier has the problem of intrinsic offset, and the intrinsic offset is always in the range of several mV to tens of mV. For wireless or wired communication, the maximum gain of variable gain amplification is up to tens of dE; therefore, the intrinsic offset after amplification will affect the recovery ability of the received signal, the characteristics of parameters of a dynamic range, and signal-to-noise ratio.
A DC offset canceling circuit is shown in FIG. 1, disclosed by Yao et al., in xe2x80x9cDC offset canceling circuit applied in a variable gain amplifierxe2x80x9d U.S. Pat. No. 6,407,630 B1. In FIG. 1, a DC offset circuit 26 applied in a variable gain amplifier 25. The variable gain amplifier 25 includes a first amplifier 21, a second amplifier 22, a plurality of switches 201xcx9c208, and a plurality of resistors. The DC offset canceling circuit 26 includes a transconductance amplifier 23 and at least one internal capacitor 24. The switches 201xcx9c204 adjust the variable gain of the first amplifier 21. For example, if the switch 201 is closed, the gain is raised; and if the switch 202 is closed, the gain is reduced. The switches 205xcx9c208 adjust the variable gain of the first amplifier 22. For example, if the switch 205 is closed, the gain is raised; and if the switch 207 is closed, the gain is reduced. The transconductance amplifier 23 is used to transform the output voltage of the second amplifier 22 to an output current based on a ratio.
The output of the transconductance amplifier 23 is coupled to at least one internal capacitor 24, and is then fed back to the input of the first amplifier 21 to cancel the DC offset-of the variable gain amplifier 25. The transconductance amplifier 23 cooperates with the internal capacitor 24, only about 10 pF or even under 10 pF, as a Gm-C filter. Since the capacitance of the internal capacitor 24 is small, the internal capacitor 24 can be manufactured easily inside an IC, and does not occupy I/O pin.
The DC offset of the first amplifier 21 and the second amplifier 22 is canceled by the transconductance amplifier 23 and capacitor 24, the Gm-C filter, but the DC offset of the transconductance amplifier 23 is not. There is a need for a novel canceling circuit to cancel the DC offset of the final stage.
According to the prior art, an extremely large chip area is required for implementing the transconductance amplifier so as to reduce the DC offset. However, the DC offset can be reduced by a chopper to saving the chip area according to the present invention described as follows.
It is therefore an object of the present invention to provide a DC offset canceling circuit of a variable gain amplifier.
To achieve the above objects, the present invention provides a DC offset canceling circuit including a transconductance amplifier, at least one internal capacitor, and chopper circuits.
A first chopper circuit 30 is inserted between the output of the variable gain amplifier and the input of the transconductance amplifier. A second chopper circuit is inserted between the output of the transconductance amplifier and the capacitor. The first chopper circuit and the second chopper circuit are controlled by a non-overlap clock signal having a chopping frequency. The first chopper circuit can be merged into the input of transconductance amplifier. The second chopper circuit can be merged into the output of transconductance amplifier.
The DC offset and low frequency noise of the transconductance amplifier, the undesired signal, is translated up to the chopping frequency. The spectrum of the undesired signal is folded back around the chopping frequency. The chopping frequency is much higher than the desired signal bandwidth, thus the size of the undesired signal in the passband of the signal is greatly reduced.
Being chopper-stabilized, the transconductance amplifier and capacitor serve the same function, canceling the DC offset of the variable gain amplifier. The chopper circuit cancels the DC offset of the transconductance amplifier.